The present invention relates to a process for forming magnetic memories and, in particular, to a process for forming magnetic disk memories using coating transfer techniques.
Until recently, magnetic disk memories were usually fabricated using web coating or spin coating techniques to apply the magnetic coating to a supporting substrate. Probably the preferred present technique for making flexible or floppy disks involves application of a predetermined thickness of magnetic particulate dispersion onto a flexible substrate of polyethylene terephthalate (polyester) using a web coating process. Typically, the composite coating-substrate web is subjected to a magnetic field to eliminate magnetic orientation within the coating in the direction of the web. The moving coating is then cured prior to the coating-web composite being formed or cut into disks. To form rigid disks, the ferric oxide particulate dispersion is spin-coated to a predetermined thickness, typically onto an aluminum alloy disk, then is oriented by spinning in a magnetic field, and cured.
Only recently have coating transfer techniques been applied to magnetic memory fabrication, as one approach to provide the necessary production volume requirements in the face of the increasingly stringent, physical and magnetic constraints which are imposed as storage densities have been increased. Coating transfer technology has been used as a vehicle for incorporating electron beam-curing and other sophisticated types of apparatus into the process flow. In short (as evidenced by the articles discussed below), the emphasis has been on complicated processing techniques.
"Transfer Coating by Electron Initiated Polymerization", paper B-4 presented by Sam V. Nablo at the 1983 Symposium on Magnetic Medium Manufacturing Methods (SM-4) traces the development of continuous or transfer processes which use electron beam (EB) curing. The Nablo paper discusses the use of direct transfer processes to form magnetic coatings for rigid disks and floppy disks. (In considering the application of such processes to disks, it will be helpful to refer to the surface or side of the magnetic coating which is adhered to the disk as the "disk side" and that which faces the magnetic head as the "head side".) Nablo indicates that the magnetic coating for rigid disks can be formed on a temporary polyester carrier film which remains in place as a protective layer until testing. In this process, the disk side of the magnetic coating is contacted by and replicated off the transfer roll, while the head side is replicated off the temporary carrier film. The article suggests that this transfer process may provide high quality surface finishes without using very stringent aluminum disk surface finishing operations such as diamond turning and post-turning lapping, if optical grade polyester is used as the temporary carrier film.
The Nablo paper also suggests two possible approaches for applying EB-cured continuous transfer technology to flexible media: the magnetic coating can be formed onto the roll or the roll may serve merely as the surfacing support. Presumably in either approach electron beam exposure cures the magnetic coating next to the drum to perfectly replicate the drum surface in the magnetic coating.
The above paper's concern with achieving near-perfection in magnetic media surface finishes reflects just one of the increasingly stringent constraints, mentioned previously herein, which are imposed in attempting to increase magnetic information storage density. The object in increasing density is, of course, to provide more information storage capacity on a disk (or a tape) of a particular size or to reduce disk size while retaining or increasing storage capacity. It is probably an industry concensus that the bit densities of longitudinally oriented systems can be increased beyond the present values of 8000-9000 bits/inch only if (1) the magnetic media thickness and the head-to-media interface (which is limited by the surface finish of the magnetic media) can be scaled to very small values and if (2) the problem of demagnetizing fields is successfully overcome. The head-to-media interface and magnetic quality are primary constraints which lead to the need for surface perfection. Unfortunately, the present longitudinal technology may be at its limits. It may not be possible to achieve more extreme scaling and greater surface perfection without the development of new materials and technology.
In contrast to longitudinal systems, perpendicular systems have density characteristics which are essentially independent of media thickness. Perpendicular systems thus do not require the extreme scaling of longitudinal systems. In addition, perpendicular magnetization is not subject to the demagnetizing fields which are present in longitudinal systems. However, the development of commercial perpendicular systems is constrained by the lack of materials which possess suitable characteristics.
Perhaps the only previously known process which uses conventional oxide particulate dispersions, yet is alleged to be suitable for high quality, high density, perpendicularly-oriented magnetic disks is the unitized transfer process disclosed in "Perpendicularly Oriented Pigmented Media", paper MMS-C, presented by Dennis E. Speliotis and Lawrence B. Lueck at the above mentioned SM-4 Symposium. Like Nablo, Speliotis et al use both transfer and EB-curing techniques. However, Speliotis et al believe that an uncured web system (which Nablo uses) may experience orientation distortion which makes it unacceptable for application to tape systems. Thus, their system was developed for unitized production and is reported to be suitable for producing both floppy and rigid disks.
Referring now to FIG. 1, the relatively complicated laminating structure used by Speliotis et al comprises in order a highly lapped transparent casting receptor 1 having a surface finish of about 0.4 microinches, a thin fluorocarbon release lubricant layer 2, and a wet magnetic dispersion coating 3 which consists of a high solids dispersion of chromium dioxide pigment in resin binders and solvents. The binders are chosen to have a large viscosity-temperature gradient. The dry coating is maintained at an elevated temperature of perhaps 60.degree.-70.degree. C. during processing to provide mobility for the magnetic pigment in the 100 percent solids dispersion. The coating 3 is oriented perpendicularly by exposure to a magnetic field from beneath the receptor 1. This assures maximum surface magnetization at the receptor-coating interface, that is, the head side of the coating. Next, the composite comprising polyester substrate 4, permalloy layer 5, and EB-curable resin laminating layer 6 is mechanically attached to the receptor-coating composite. This assembly is exposed to electron beam radiation from below to cure the laminating layer 6 and thereby laminate the assembly. The assembly is then delaminated from the receptor. The authors attribute to this process and structure the advantages of perpendicular orientation, replication of the receptor surface perfection in the head side of the magnetic coating, and avoidance of the so-called Manhattan vista effect in the magnetic particle profile.
At this point, it should be apparent that the application of coating transfer technology to the production of magnetic memory media is characterized by the use of sophisticated expensive equipment, such as electron beam generating apparatus, and by relatively complicated processes and media structures. To the knowledge of this inventor, to date no one else has developed a simple coating transfer process which provides excellent thickness uniformity and range and quality and which is also suitable for use in high volume commercial production. Indeed, the trend seems to be the opposite, toward increasingly complicated vacuum deposition, electron beam and other sophisticated processes.